1. Field of the Invention
The present invention relates to a method and system for synchronizing downlink and uplink signals between a satellite and satellite terminals in a satellite communication system. More particularly, the present invention relates to a method and system for synchronizing downlink and uplink signals between a satellite and satellite terminals in a satellite communication system using a discontinuous synchronization signal embedded within frames of the downlink signal.
2. Description of the Related Art
Society has an ever increasing appetite for the exchange of information. A number of communication systems exist which attempt to satisfy society's communications needs. A useful communication system should be reliable, inexpensive and available to a wide percentage of the population, even in geographically remote areas. Existing communication systems each have disadvantages. For example, digital subscriber line (xDSL) services have been proposed to accommodate the transport of digitized voice and data on analog telephone lines. However, difficulties have been experienced with insuring that xDSL services consistently deliver the bandwidth that is requested by users.
In addition, large populations exist that do not have access to plain old telephone service (POTS). Furthermore, even where POTS is available, xDSL services may not be available because of the distance between a consumer and a central office, or because of problems with noise on the analog telephone line. Some cable companies offer high speed internet services over existing cable networks. However, access to cable internet service, like DSL, is limited to geographic regions where the infrastructure exists. Furthermore, it would be extremely expensive to build the infrastructure for telephone or cable service in such geographically remote areas.
Other examples of communication systems include wireless networks to provide for the transmission of packetized data over cellular voice networks, personal communication systems (PCS), and point-to-multipoint systems for broad-band wireless network access. These systems are disadvantageous in that they limit users delivery options. For example, cellular voice networks are limited to voice communications and personal communication systems provide access to either very limited information or provide internet access at relatively slow data rates compared to even dial-up connections. Furthermore, cellular and PCS systems are still geographically limited to locations where the cellular infrastructure exists.
Satellite communication systems are advantageous because they can serve an extremely wide geographic region. For example, a single geosynchronous satellite may service the entire North American continent. Very small aperture terminal (VSAT) satellite networks provide business enterprises and other organizations with local area network (LAN) internetworking, batch and interactive transmission service, interactive voice, broadcast data and voice communications, multimedia image transfer service, and other services, between a number of sites equipped with VSATs and a site designated as their headquarters. Some existing VSAT satellite networks, however, are disadvantageous in that they typically use large antennas, require double satellite hops through a central hub for VSAT to VSAT data transfers, and transmit and receive at relatively low data rates. Other satellite systems provide only push internet service to consumers (i.e. access to selected information available via internet) and not full access to all internet information and full connectivity.
There is therefore a need for a satellite communication system that overcomes the above-listed disadvantages. Such a system should provide broadband multimedia services to an individual or entity within the geographic area covered by the satellite. In the case of a geosynchronic satellite, customers in the northern hemisphere should require only a clear view of the southern sky and a satellite terminal capable of receiving from and transmitting to the satellite.
Two very important considerations in a two-way satellite communication system will be the system's capacity and the cost of the satellite terminals. The capacity of the system is determined by the frequency band allocated to the system. For Ka band Fixed Satellite Services, a contiguous spectrum of 500 MHz is typically allocated for the downlink as well as the uplink. The capacity of the system is increased by dividing the coverage area into geographically distinct uplink and downlink cells. Multiple modulators and beam shaping is utilized on the satellite to limit the coverage of each beam to a particular cell or group of cells. In this manner, the allocated spectrum may be reused in geographically distinct areas. However, using multiple modulators increases the complexity of a satellite. Therefore, there is a need to reduce the complexity of the satellite where possible.
In addition, the cost of satellite terminals (ST) should be kept to a minimum. Because many STs will be present within each uplink and downlink cell, each uplink cell is typically assigned to a particular sub-band of the allocated spectrum, and each ST within the uplink cell is typically assigned to a particular time slot. Thus, it is critical to the functioning of the system for the STs to be synchronized in both timing and frequency with the satellite. Traditional satellite systems incorporate a beacon signal on a separate carrier frequency in order to synchronize the ST with the satellite. However, providing a beacon signal on a separate carrier requires an additional modulator on the satellite and additional hardware for demodulating at the ST. This adds unwanted cost and complexity to the system. Therefore, there is a need to provide a means for synchronizing STs with the satellite to a high degree of accuracy while at the same time reducing the cost and complexity of the STs and the satellite.